fix: reject unsupported URL schemes

[PragnyaKhandelwal]

Summary

• Reject unsupported URL schemes (e.g., ftp://) before file-path fallback
• Add regression test for unsupported schemes

Linked issue

Fixes docling-project/docling#3396

Testing

• pytest test/test_utils.py

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Thanks @PragnyaKhandelwal, all your commits are properly signed off. 🎉

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[PragnyaKhandelwal]

Hi team! Just a gentle nudge on this PR as the new week starts. It addresses the URL scheme crash in #3396 and includes regression tests.

I noticed the GitHub Actions are currently awaiting maintainer approval to run—would appreciate it if someone could approve the workflows so the CI can verify the changes! Thanks!

[PragnyaKhandelwal]

Thanks for the approval, @sasi22498!
It looks like Mergify requires a second reviewer since this PR includes test updates. Additionally, the CI workflows are still awaiting a maintainer's approval to run.
Hey @PeterStaar-IBM could you help trigger the CI checks and provide the second review when they have a moment? Thanks!